#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <cmath>
#include <climits>
#include <iomanip>
#include <set>
#include <map>
using namespace std;
typedef long long ll;
int main(){
    ll a,b,c,d;
    cin >> a >> b >> c >> d;
    double res = abs(c - a) + abs(d - b);
    res /= (double)2;
    cout << fixed << setprecision(10) << (double)res << endl;
}